This invention relates to improved aqueous based pressure sensitive adhesive emulsions, the improvement residing in the incorporation of polymerized units of a vinyl ester of a neo-acid into the copolymer present in these emulsions.
Pressure sensitive adhesives are widely used for fabricating paper labels, laminating polymeric films such as laminating poly(vinyl chloride), polyester, etc., for forming decals and other related products. Most of the pressure sensitive adhesives have been solvent based but in recent year government regulations have forced the industry to develop environmentally friendly aqueous based pressure sensitive adhesives.
Aqueous based pressure sensitive adhesives comprised of polymerized monomer units of vinyl acetate, ethylene, and/or acrylic esters, etc. have been inferior to the solvent based pressure sensitive adhesives in that they do not have sufficient adhesive power to low surface energy substrates nor do they have good water resistance and so forth.
The following patents describe a variety of emulsion polymers incorporating vinyl esters and/or emulsion based pressure sensitive adhesives:
U.S. Pat. No. 5,461,103 discloses aqueous based pressure sensitive adhesive emulsions having an ability to coat low energy surfaces without retracting or cratering. The emulsion polymers are prepared by a two stage emulsion polymerization process, the first stage resulting in the formation of an alkali insoluble polymeric system incorporating acid functional groups and polymerized hydrophobic monomer units, e.g., C4-12 alkyl acrylates and the second stage incorporating monomer units suited for the manufacture of pressure sensitive adhesives, e.g., acrylic and methacrylic esters, vinyl esters of C2-5 carboxylic acids, e.g., vinyl acetate and vinyl propionate and the like.
U.S. Pat. No. 4,923,919 discloses pressure sensitive adhesive emulsions based on acrylic polymers that contain at least 40% by weight of an alkyl acrylate and a support resin that is alkali soluble. The support resin contains polymerizable carboxylic acid anhydride groups with the polymeric resin having a umber average molecular weight of from 1000-15,000. Typically the support resins are based on vinyl monomers such as acrylic and methacrylic acid esters, styrene and combinations thereof.
U.S. Pat. No. 4,322,516 discloses aqueous based pressure sensitive adhesives formed from ethylene units, acrylic ester units, vinyl acetate units, acrylamide units and other olefinically unsaturated monomers. Tackifying agents are incorporated into the ethylene based pressure sensitive adhesives.
U.S. Pat. No. 4,939,190 discloses emulsion type pressure sensitive adhesives wherein the emulsions are prepared in a staged procedure, the first stage comprising polymerizing an ethylenically unsaturated monomer such as styrene, acrylonitrile and/or (meth)acrylates in the presence of a tackifier resin and a second stage comprising polymerizing alkyl acrylates having at least 4 carbon atoms in the presence of the first stage polymer.
U.S. Pat. No. 5,276,084 discloses aqueous based pressure sensitive adhesives based on polymers comprising vinyl acetate, ethylene, alkyl acrylates dialkyl maleate and fumarates, hydroxyalkyl acrylates, acrylamides and unsaturated carboxylic acids such as acrylic acid.
U.S. Pat. No. 3,697,618 discloses acrylic pressure sensitive adhesives incorporating a vinyl ester of a carboxylic acid, e.g., vinyl acetate, a lower alkyl acrylate and a copolymerizable alpha, beta-ethylenically unsaturated dicarboxylic acid or anhydride.
U.S. Pat. No. 3,562,229 discloses solvent polymerized copolymers of ethylene and vinyl esters where the vinyl esters are derived from long chain carboxylic acids, e.g., versatic acid which are mixtures of C9-11 carboxylic acids. Isotridecanoic and isononanoic are representative acids. The solvent based polymers incorporating vinyl esters of versatic acid were alleged to be superior to polymers incorporating vinyl laurate which has the same number of carbon atoms.
U.S. Pat. No. 3,294,727 discloses polymerizing vinyl esters of saturated monocarboxylic acids to form emulsions having suitability for producing freeze-thaw stable paints. The preferred saturated aliphatic monocarboxylic acids for forming vinyl esters are based upon compounds of the formula: 
wherein R1 and R2 are alkyl radicals of normal, branched, or cyclic structure and R3 represents a hydrogen or hydrocarbyl radical. The carboxylic acids have from about 9-19 carbon atoms. Representative copolymers are comprised of polymerized units of vinyl acetate and/or acrylic acid and the representative vinyl esters.
U.S. Pat. No. 3,669,942 discloses solvent based polymers comprising a vinyl ester of an alpha-branched, saturated aliphatic monocarboxylic acid having from 5-20 carbon atoms and an alpha, beta-ethylenically unsaturated carboxylic acid having 3-5 carbon atoms, a monovinyl aromatic compound and acrylic acid esters including hydroxyalkyl acrylate esters. The compositions are useful as water thinnable paint binders.
U.S. Pat. No. 3,654,213 discloses tacky pressure sensitive adhesives based on interpolymers of vinyl esters and alkyl acrylates, the interpolymers being dissolved at high solids content in organic solvents. Representative interpolymers are based upon vinyl acetate, vinyl propionate, vinyl butyrate, and vinyl valerate with vinyl acetate being preferred. Other monomers which can be incorporated into the polymer include acrylic acid esters such as methyl acrylate, ethyl acrylate and so forth.
U.S. Pat. No. 4,322,516 discloses pressure sensitive adhesive emulsions whose copolymers comprise:
(a) 10-30 wt % ethylene,
(b) 29-69 wt % acrylic aciesters of alcohols,
(c) 20-55 wt % vinyl acetate,
(d) 0.2-8 wt % (meth)acrylamide, and
(e) 0-12 wt % other olefinically unsaturated comonomers.
The present invention is directed to improved aqueous based pressure sensitive adhesive emulsion polymers. Conventional aqueous emulsion based pressure sensitive adhesives are comprised of a copolymer comprised of polymerized units of acrylic esters and ethylenically unsaturated monomers, the copolymer having a Tg of from xe2x88x9215 to xe2x88x9270xc2x0 C. The improvement in the aqueous based, emulsion polymerized pressure sensitive adhesives resides in the incorporation of a vinyl ester of a C8-13 neo-acid into the copolymers present in the pressure sensitive adhesives. Such copolymers incorporate a vinyl ester formed from a mixture of propylene oligomers and such vinyl ester is represented by the chemical formula:
C8-13H17-27CO2CHxe2x95x90CH2
There are significant advantages associated with the above aqueous based pressure sensitive adhesive emulsions and the process for producing these emulsions and these advantages include:
an ability to provide enhanced properties to pressure sensitive adhesives through the methyl group rich vinyl aster mixture which exhibits a lower solubility parameter compared to t he corresponding linear alkyl vinyl esters;
an ability to provide enhanced properties to pressure sensitive adhesives through the ability of the vinyl ester monomer to undergo emulsion polymerization with a variety of olefincally unsaturated comonomers;
an ability to form an excellent bond and enhanced resistance to plasticizer migration with plasticized poly(vinyl chloride);
an ability to generate a class of pressure sensitive adhesives which have an excellent balance of peel and cohesive strength;
an ability to provide enhanced thermal and hydrolytic stability to emulsion based pressure sensitive adhesives is compared to aqueous based vinyl acetate counterparts; and,
an ability to provide enhanced properties to pressure sensitive adhesives through binding to lower surface energy surfaces of lower polarity, such as polyethylene, by virtue of stronger bonding of the copolymers
Emulsion polymerization of ethylenically unsaturated monomers such as lower alkyl acrylates and vinyl esters, typically represented by vinyl acetate or vinyl propionate, to produce aqueous based pressure sensitive adhesive emulsions containing vinyl copolymer is well known. The improvement to emulsion based pressure sensitive adhesives containing vinyl copolymer incorporating polymerized units of ethylenically unsaturated monomer units resides in the incorporation of a vinyl ester of a neo-acid derived from a propylene oligomer feedstock into the vinyl copolymer.
The vinyl ester of a neo-acid of propylene oligomers or mixture thereof is incorporated into the vinyl polymer is comprised of the vinyl ester of a single or mixture of tri- and tetramers of propylene which have been converted to the corresponding single or mixture of C8-13 neo-acids. These esters typically are prepared by first converting the tri and tetramers of propylene to the corresponding acids and then converting the corresponding those neo-acids to the vinyl esters. The vinylation of those acids can be achieved either by transvinylation with vinyl acetate or by catalytic addition of the neo-acids to acetylene. Some of the vinyl esters may be represented by a singular neo-acid, but generally the vinyl esters of the neo-acids are present as a mixture.
Aqueous based pressure sensitive adhesive emulsions are based on emulsions which comprise a colloidal dispersion of the vinyl copolymer in water. The copolymers present in the pressure sensitive adhesives are typically derived from the following monomers and within the percentage proportions listed below:
(a) 5-40 wt % of a vinyl ester of a C8-13 neo-acid, preferably 15 to 30 wt %;
(b) 30-80 wt % of a C1-12 alkyl ester of acrylic or methacrylic acid, preferably 40 to 70 wt %,
(c) 0-20 wt % of a vinyl ester of saturated aliphatic acids, preferably 0 to 10%;
(d) 0-30 wt % ethylene, styrene or butadiene, preferably 0 to 20 wt %;
(e) 0-20 wt % di-(C1-3)alkyl maleate/fumarate, preferably 0 to 10 wt %;
(f) 0-5 wt % of a hydroxyalkyl (meth)acrylate;
(g) 0-5 wt % (meth)acrylamide; and,
(h) 0-10 wt % alpha, beta-ethylenically unsaturated monocarboxylic acid.
A variety of other comonomers, eg., other ethylenically unsaturated monomers sometimes employed in the formation of the aqueous based pressure sensitive emulsions can be copolymerized with the vinyl ester of a neo-acid to form adhesives. For example, C1-18 alkyl vinyl ethers, vinyl chloride, vinylidene chloride, nitriles, carboxylic amides such as N-vinylformamide and N-vinylacetamide, acrylonitrile and methacrylonitrile to produce a variety of polymers. Examples of useful alkyl vinyl ethers are methyl vinyl ether, ethyl vinyl ether, isopropyl vinyl ether, n-propyl vinyl ether, tert-butyl vinyl ether and n- and isobutyl vinyl ether.
Component (a) monomers which are the vinyl esters of C8-13 neo-acids and utilized in producing the improved pressure sensitive adhesives have been defined.
Component (b) monomers utilized in producing the pressure sensitive adhesives containing the vinyl esters of C8-13 neo-acids are alkyl esters of acrylic or methacrylic acid. These are esters of acrylic acid or methacrylic acid and a C1-12 alcohol. Examples of suitable alcohols for the preparation of the esters are methanol, ethanol, propanol, isopropanol, n-, iso- and tert-butyl alcohol, neopentyl alcohol, 2-ethyl-hexanol, n-octanol, dodecanol, palmityl alcohol and stearyl alcohol. Specific examples of unsaturated alkyl acrylate monomers include methyl methacylate, ethyl acrylate, butyl acrylate, hexyl acrylate 2-ethylhexyl acrylate, isooctyl acrylate, isodecyl acrylate and the like.
Component (c) monomers which may be used in forming emulsion polymers for pressure sensitive adhesives can include other vinyl esters than the vinyl esters of neo-acids. Some of the more common vinyl esters used in the polymer synthesis include C3-18 alkanoic acids, such as vinyl acetate, vinyl propionate, vinyl butyrate, vinyl laurate and the like. These esters, and particularly the lower alkyl esters, may detract from the overall properties of the pressure sensitive emulsion imparted by the vinyl ester of the C8-13neo-acids.
Component (d) monomers used as fraction of the polymers utilized in forming emulsion polymers for pressure sensitive adhesives can include hydrocarbon monomers such as ethylene, styrene, butadiene.
Component (e) monomers used in forming many of the pressure sensitive adhesives include an alkyl maleate or fumarate, for the purpose of providing enhanced tack. etc. One may suitably use the diesters of maleic acid or fumaric acid, the diester being formed by the reaction of the acid with a C1-13, such as, n-octyl alcohol, isooctyl alcohol, butyl alcohol, isobutyl alcohol, methyl alcohol, amyl alcohol and the like. Preferably a C4-8, alcohol is employed.
Components (f) monomers often ere incorporated into the pressure sensitive adhesive polymer. Representative monomers include hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylates, hydroxypropyl methacrylate, hydroxypropyl acrylates, hydroxypropyl methacrylates, hydroxybutyl acrylates and hydroxybutyl methacrylates.
Component (g) monomers often are incorporated into the copolymers at some levels. Examples of amides include acrylamide, methacrylamide and N-methylol-acrylamide.
Component (h) monomers include the alpha, beta-ethylenically unsaturated C3-18 carboxylic acids which can be incorporate into the polymer can be methacrylic acid, crotonic acid, acrylic acid and the like.
The sum of the monomers used to form the pressure sensitive copolymers is 100 percent. The emulsion copolymers are designed to have a Tg of less than xe2x88x9215xc2x0 C., preferably xe2x88x9215 to xe2x88x9270xc2x0 C. and typically xe2x88x9220 to xe2x88x9250xc2x0 C. The aqueous emulsions contain about 40 to 70 wt % solids.
The emulsion polymerization process typically involves an initial homogenization in which the monomers which include the vinyl esters of neo-acids and the other monomers which are employed to form the vinyl copolymer such as C1-12 alkyl acrylates or methacrylates, vinyl acetate, dialkyl maleate/fumarate, ethylene, acrylamide and other monomers are thoroughly agitated. If ethylene is added to the reactor to form a copolymer, the reactor is pressured to the working pressure to effect solution of the ethylene in the monomers. Then, the reaction medium is gradually heated to polymerization temperature. The homogenization period is followed by a polymerization period during which the free radical source is added incrementally.
Various free-radical forming catalysts such as peroxide compounds can be used in carrying out the emulsion polymerization of the monomers. Combination-type catalysts employing both reducing agents and oxidizing agents can also be used. The use of this type of combined catalyst is generally referred to in the art as xe2x80x9credox polymerizationxe2x80x9d or xe2x80x9credox system.xe2x80x9d The reducing agent is also often referred to as an activator and the oxidizing agent as an initiator. Suitable reducing agents or activators include bisulfites, sulfoxylates, or other compounds having reducing properties such as ferrous salts, and tertiary aromatic amines, e.g., N,N-dimethylaniline. The oxidizing agents or initiators include hydrogen peroxide, organic peroxides such as benzoyl peroxide, t-butyl hydroperoxide and the like, persulfates, such as ammonium or potassium persulfate, perborates, and the like. A specific combination-type catalyst or redox system which can be used is hydrogen peroxide and sodium formaldehyde sulfoxylate.
The initiator is employed in the amount of 0.1 to 2%, preferably 0.25 to 0.75%, based on the weight of monomers introduced into the reactor. The activator is ordinarily added as an aqueous solution and the amount of activator is generally from 0.25 to 1 times the amount of initiator.
A wide variety of nonionic, anionic and cationic surfactants can be used to stabilize the emulsion in emulsion polymerization processes. Suitable nonionic emulsifying agents include polyoxyethylene condensates. Polyoxyethylene condensates may be represented by the general formula:
R (CH2xe2x80x94CH2xe2x80x94O)nH
where R is the residue of a fatty alcohol containing 10-18 carbon atoms, an alkylphenol, a fatty acid containing from 10-18 carbon atoms, an amide, an amine, or a mercaptan, and where n is an integer of 1 or above. Some specific examples of polyoxyethylene condensates which can be used include polyoxyethylene aliphatic ethers such as polyoxyethylene lauryl ether, polyoxyethylene oleyl, ether, polyoxyethylene hydroabietyl ether and the like; polyoxyethylene alkaryl ethers such as polyoxyethylene nonylphenyl ether, polyoxyethylene octylphenyl ether and the like; polyoxyethylene esters of higher fatty acids such as polyoxyethylene laurate, polyoxyethylene oleate and the like as well as condensates of ethylene oxide with resin acids and tall oil acids; polyoxyethylene amide and amine condensates such as N-polyoxyethylene lauramide, and N-lauryl-N-polyoxyethylene amine and the like; and polyoxyethylene thioethers such as polyoxyethylene n-dodecyl thioether. Other suitable nonionic surfactants are alkylarylethoxylate alcohols, alkylethoxylate alcohol, ethoxypropoxylate, alcohols from ethylene oxide and/or propylene oxide reaction products, and the like.
Another class of nonionic surfactants are sold under the Igepal trademark. One example within this class is a polyoxyethylene nonylphenyl ether having a cloud point of between 126 and 133xc2x0 F. and marketed under the trade mark xe2x80x9cIgepal CO-630xe2x80x9d; another is polyoxyethylene nonylphenyl ether having a cloud point above 212xc2x0 F. and marketed under the trade mark xe2x80x9cIgepal CO-887.xe2x80x9d
Examples of anionic surfactants include metal or ammonium salts of long chain alkyl sulfates and the likes; metal or ammonium salts of alkylarylethoxylated sulfates or sulfonates, paraffin sulfonates, alkyl sulfosuccinates, alkylarylsulfonates, alkyl phosphates, etc.
A protective colloid also can be used in the polymerization mixture as a stabilizing agent. Various colloids and amounts conventionally used in emulsion polymerization can be incorporated into the latices as desired and in combination with the surfactants. Representative colloids which can be used include poly(vinyl alcohol), partially-acetylated poly(vinyl alcohol), e.g. up to 50% acetylated, casein, hydroxyethyl starch, carboxymethylcellulose, gum arabic, and the like.
The concentration range of the total amount of emulsifying agents used in emulsion polymerization is from 0.1 to 10% based on the aqueous phase of the latex regardless of the solids content. The stabilizers employed are, in part, governed by the use to which the copolymer latex is to be put. By utilizing appropriate levels of surfactant and/or protective colloid, one can obtain latex polymer particles having a variety of average particle size ranges and distributions.
In order to maintain the pH of the system at the desired value, there is suitably added a buffering agent of any convenient type. Any alkaline material which is compatible with the stabilizing agent can be used as the buffer. The amount of buffer is that sufficient to adjust the pH of the system within the desired range, e.g., 2.5 to 10 and preferably 3.5 and 8.5. The amount of buffer is generally about 0.1 to 0.5% by weight, based on the monomers.
Reaction temperatures for emulsion polymerizing the vinyl esters of neo-acids along with other monomers are conventional. The reaction temperature can be controlled by the rate of catalyst addition and by the rate of the heat dissipation therefrom. Generally, it is advantageous to maintain a temperature from about 50 to 90xc2x0 C. While temperatures as low as 0xc2x0 C. can be used, economically, the lower temperature limit is about 40xc2x0 C.
The reaction time will also vary depending upon other variables such as the temperature, the catalyst, and the desired extent of the polymerization. It is generally desirable to continue the reaction until le s than 0.1%. of the vinyl ester remains unreacted. Under these circumstances, a reaction time of about 4 hours has been found to be generally sufficient for complete polymerization, but reaction times ranging from 2 to 10 hours have been used, and other reaction times can be employed, if desired.
The following examples are provided to illustrate various embodiments of the invention and are not intended to restrict the scope thereof.